Practice golf ball

ABSTRACT

A practice golf ball has a core and a cover. The core is made of a rubber composition which includes a base rubber, a co-crosslinking agent, a crosslinking initiator and a metal oxide. The base rubber is a mixture of polybutadiene with styrene-butadiene rubber and/or isoprene rubber, and the co-crosslinking agent is methacrylic acid. The core has a specific deflection under a specific load, and the ball has a specific initial velocity. The dimples on the surface of the ball have a volume ratio VR of from 0.95 to 1.7.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2011-099969 filed in Japan on Apr. 27, 2011, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a practice golf ball which is suitable for use at such places as driving ranges. More specifically, the invention relates to a practice golf ball which is endowed with the properties required of practice balls intended for long-term use, including better durability to cracking and durability of appearance than ordinary game balls and the ability to maintain a stable feel on impact and a stable flight performance over a long period of time, and which, moreover, by further limiting the flight of the ball, has a controlled distance for use at very small driving ranges.

2. Prior Art

In terms of the performance sought in a practice golf ball, it is generally desirable to obtain ball characteristics which are no different from those of golf balls used to play an actual round of golf. If, for example, a practice ball has a feel on impact or a flight performance which differs from that of a game ball, when the time comes to play an actual round, the golfer will be unable to take full advantage of the skills acquired through practice.

On the other hand, because of the size limitations of driving ranges, there is a need today for low-distance practice golf balls in order to keep the balls from flying out of the driving range. Accordingly, there exists a desire for practice balls which have a performance characterized by the same feel on impact as a game ball, but a short flight distance.

Moreover, because practice golf balls are used over and over again at a driving range, in order to reduce the costs of operating the range, it is desirable for the balls to be capable of enduring use for as long a period of time as possible even when repeatedly hit. That is, because the golf balls at a driving range are repeatedly used over a long period of time by many golfers practicing their skills, there is a desire for such practice balls to have a durability, including a durability to cracking, which is superior to that of ordinarily used game balls.

As for the ball structure in a practice golf ball, for the ball to be imparted with a flight performance and feel similar to those experienced when playing an actual round of golf, it is more desirable to use a two-piece solid golf ball than a one-piece ball.

As is widely known, two-piece solid golf balls are composed of a core and a cover, with the core being a rubber crosslinked material of certain desirable properties obtained by using a base rubber composed primarily of cis-1,4-polybutadiene rubber to which compounding ingredients such as a co-crosslinking agent, a metal oxide and an organic peroxide have been added. For example, JP-A 59-49779 teaches, as the rubber composition for the core of a two-piece solid golf ball, the compounding of a given amount of zinc methacrylate as a co-crosslinking agent in cis-1,4-polybutadiene rubber. However, when zinc methacrylate is used in this way in a core-forming rubber composition, achieving good ball durability in long-term use at a golf driving range has been difficult.

In addition, JP-A 2003-70936 describes, as a rubber composition for the core of a two-piece solid golf ball, the compounding of a given amount of zinc acrylate in cis-1,4-polybutadiene rubber. However, here too, when zinc acrylate is used in the rubber-forming rubber composition, achieving good ball durability in long-term use at a driving range has been difficult.

Also, JP-A 2004-180793 and JP-A 2008-149190 disclose golf balls which use zinc acrylate in the core formulation and use a thermoplastic polyurethane as the cover material. However, drawbacks of such golf balls include a hard cover, a large decrease in ball flight following abrasion of the ball surface, and poor durability of markings (“markings” referring here to, for example, a brand name or player number printed on the surface of the ball at the time of manufacture).

In view of the foregoing, it is an object of the present invention to provide a practice golf ball which is endowed with the properties required of practice balls intended for long-term use, including better durability to cracking and durability of appearance than ordinary game balls and the ability to maintain a stable feel on impact and a stable flight performance over a long period of time, and which moreover, by further limiting the flight of the ball, has a controlled distance for use at very small driving ranges.

SUMMARY OF THE INVENTION

We have discovered that, in the production of practice golf balls having a core made of a rubber composition and a cover encasing the core, by using a mixture of polybutadiene with styrene-butadiene rubber and/or isoprene rubber as the base rubber, by using methacrylic acid as a co-crosslinking agent in the rubber composition and also including a given amount of a crosslinking initiator per 100 parts by weight of the base rubber, and moreover by optimizing the core deflection under a specific load, the initial velocity of the finished ball and the dimple volume ratio VR, owing to the synergistic effects of these features, the resulting balls have better durability to cracking and durability to abrasion than anticipated by golf ball designers. This discovery has made it possible to obtain a practice golf ball which, even with long-term use, retains a good appearance and a good flight performance, has a good feel on impact and, by further limiting the flight of the ball, has a controlled distance.

That is, in the present invention, by using methacrylic acid as a co-crosslinking agent in the core-forming rubber composition and by optimizing the amounts of methacrylic acid and crosslinking initiator included in the rubber composition, the ball can be made much more durable to cracking than a game ball. In addition, by having the cover material formed primarily of polyurethane, a practice golf ball endowed with excellent durability to cracking and durability to abrasion can be obtained. Moreover, by optimizing the internal hardness profile of the core, a golf ball having a good feel on impact can be obtained.

Accordingly, the invention provides a practice golf ball having a core made of a rubber composition comprising a base rubber and, as compounding ingredients: a co-crosslinking agent, a crosslinking initiator and a metal oxide; and having a cover which encases the core and comprises a resin component. The base rubber is a mixture of polybutadiene with styrene-butadiene rubber and/or isoprene rubber, and the co-crosslinking agent is methacrylic acid. The core, when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), has a deflection CH of from 2.5 mm to 7.0 mm. The ball has an initial velocity of not more than 70 m/s. The ball has formed on a surface thereof a plurality of dimples, each dimple having a spatial volume below a flat plane circumscribed by an edge of the dimple, and the sum of the dimple spatial volumes, expressed as a percentage (VR) of the volume of a hypothetical sphere representing the ball were the ball to have no dimples on the surface thereof, being from 0.95% to 1.7%.

Preferably, in the practice golf ball of the invention, the core has a hardness profile in which, letting A be the JIS-C hardness at a surface of the core, B be the JIS-C hardness at a position 2 mm inside the core surface, C be the JIS-C hardness at a position 5 mm inside the core surface, D be the JIS-C hardness at a position 10 mm inside the core surface, E be the JIS-C hardness at a position 15 mm inside the core surface, and F be the JIS-C hardness at a center of the core: A is from 60 to 88, B is from 54 to 83, C is from 56 to 85, D is from 54 to 80, E is from 51 to 75, and F is from 48 to 72; the relative hardness conditions A>B<C≧D>E>F are satisfied; the value A-F is not more than 19; the core is formed in such a way that A has the highest value among A to F; and the value A-C is from 0 to 8. The core preferably has a specific gravity of from 1.05 to 1.2. The resin component of the cover is preferably composed primarily of polyurethane. The cover preferably has a material hardness, expressed as the Shore D hardness, of from 30 to 57, and preferably has a thickness of from 0.3 mm to 2.5 mm. When the core and the ball are each compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), letting deflection by the core be CH and deflection by the ball be BH1, the ratio CH/BH1 is preferably from 0.95 to 1.1.

The practice golf ball of the invention preferably has, upon initial measurement, a deflection BH1 (mm) when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) and an initial velocity BV1 (m/s) and, when measured again after being left to stand for 350 days following initial measurement, a deflection BH2 (mm) when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) and an initial velocity BV2 (m/s), such that the difference BH2−BH1 is not more than 0.2 mm and the difference BV2−BV1 is not more than 0.3 m/s.

The practice golf ball may have has formed on a surface thereof a plurality of dimples which satisfy conditions (1) and (2) below:

(1) the dimples have a peripheral edge provided with a roundness represented by a radius of curvature R of from 0.5 mm to 2.5 mm; and

(2) the ratio ER of a collective number of dimples RA having a radius of curvature R to diameter D ratio (R/D) of at least 20%, divided by a total number of dimples N on the surface of the ball, is from 15% to 95%.

The practice golf ball which satisfies above conditions (1) and (2) may satisfy also condition (3) below:

(3) the ball has thereon a plurality of dimple types of differing diameter, and the ratio DER of a combined number of dimples DE obtained by adding together dimples having an own diameter and an own radius of curvature larger than or equal to a radius of curvature of dimples of larger diameter than the own diameter plus dimples of a type having a largest diameter, divided by the total number of dimples N on the surface of the ball, is at least 80%.

The practice golf ball which satisfies above conditions (1) to (3) may satisfy also conditions (4) to (6) below:

(4) the number of dimple types of differing diameter is 3 or more;

(5) the total number of dimples N is not more than 380; and

(6) the surface coverage SR of the dimples, which is the sum of individual dimple surface areas, each defined by a flat plane circumscribed by an edge of the dimple, expressed as a percentage of the surface area of a hypothetical sphere representing the ball were the ball to have no dimples on the surface thereof, is from 60% to 74%.

The polyurethane in the resin component of the cover may be a thermoplastic polyurethane elastomer, in which case the thermoplastic polyurethane elastomer preferably includes soft segments formed from a polymeric polyether polyol and hard segments formed from an aromatic diisocyanate.

The practice golf ball of the invention preferably has, upon initial measurement, a deflection BH1 when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) of from 2.5 mm to 7.0 mm.

Preferably, in the rubber composition making up the core of the practice golf ball of the invention, polybutadiene accounts for not more than 80 wt % of the base rubber, styrene-butadiene rubber and/or isoprene rubber account for from 20 wt % to 60 wt % of the base rubber, and the compounding ingredients are included in respective amounts of from 10 to 40 parts by weight of methacrylic acid, from 15 to 30 parts by weight of metal oxide, from 0.3 to 5.0 parts by weight of crosslinking initiator, and from 0.1 to 1.0 part by weight of an antioxidant, per 100 parts by weight of the base rubber.

The practice golf ball of the invention is endowed with the properties required of practice balls intended for long-term use, including better durability to cracking and durability of appearance than ordinary game balls, the ability to maintain a stable feel on impact and a stable flight performance over a long period of time, and also, by further limiting the flight of the ball, a controlled distance for use at very small driving ranges.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a schematic cross-sectional diagram of a practice golf ball according to one embodiment of the invention.

FIG. 2 is a schematic diagram of a core illustrating positions A to F in the core hardness profile.

FIG. 3 is a schematic diagram showing an example of a dimple cross-section.

FIG. 4A is a top view and FIG. 4B is a side view showing an example of a dimple configuration.

FIG. 5 is a top view showing the markings that were placed on the golf balls fabricated in the examples and the comparative examples.

DETAILED DESCRIPTION OF THE INVENTION

The objects, features and advantages of the invention will become more apparent from the following detailed description, taken in conjunction with the foregoing diagrams.

The structure of the practice golf ball of the invention is exemplified by, as shown in FIG. 1, a two-piece solid golf ball G having a core 1 and a cover 2 which encases the core 1. The cover 2 typically has a plurality of dimples D formed on a surface thereof. In the diagram, the core 1 and the cover 2 are shown as single layers, although the core and the cover may each be composed of a plurality of layers.

The core is obtained by vulcanizing a rubber composition composed primarily of a rubber material. The rubber composition used to form the core includes a base rubber, a co-crosslinking agent, a crosslinking initiator, a metal oxide and, optionally, an antioxidant. A mixture of polybutadiene with a styrene-butadiene rubber and/or an isoprene rubber is used as the base rubber in the rubber composition. In the present invention, as will be subsequently described, it is preferable for the core cross-sectional hardness to change in specific ways from the surface to the center of the core, and for the core cross-sectional hardness distribution, also referred to herein as the “core hardness profile,” to be adjusted within certain desired ranges. To this end, in formulating the core, it is essential to suitably adjust, for example, the amounts in which the various subsequently described compounding ingredients are included, the vulcanization temperature and the vulcanization time.

The polybutadiene used as a rubber component must have a cis-1,4 bond content of at least 60 wt %, preferably at least 80 wt %, more preferably at least 90 wt %, and most preferably at least 95 wt %. If the cis-1,4 bond content is too low, the rebound may decrease. In addition, the polybutadiene has a 1,2-vinyl bond content of preferably 2 wt % or less, more preferably 1.7 wt % or less, and even more preferably 1.5 wt % or less.

The polybutadiene has a Mooney viscosity (ML₁₊₄ (100° C.)) which is preferably at least 30, more preferably at least 35, even more preferably at least 40, and most preferably at least 45, but is preferably not more than 100, more preferably not more than 80, even more preferably not more than 70, and most preferably not more than 60.

The term “Mooney viscosity” used herein refers to an industrial indicator of viscosity (JIS K6300) as measured with a Mooney viscometer, which is a type of rotary plastometer. This value is represented by the unit symbol ML₁₊₄ (100° C.), wherein “M” stands for Mooney viscosity, “L” stands for large rotor (L-type), and “1+4” stands for a pre-heating time of 1 minute and a rotor rotation time of 4 minutes. The “100° C.” indicates that measurement was carried is out at a temperature of 100° C.

In order to obtain the rubber composition in a molded and vulcanized form which has a good rebound, it is preferable for the polybutadiene to have been synthesized using a rare-earth catalyst or a Group VIII metal compound catalyst.

The rare-earth catalyst is not subject to any particular limitation, although preferred use can be made of a catalyst which employs a lanthanum series rare-earth compound. Also, where necessary, an organoaluminum compound, an alumoxane, a halogen-bearing compound and a Lewis base may be used in combination with the lanthanum series rare-earth compound. Preferred use can be made of, as the various above compounds, those compounds mentioned in JP-A 11-35633, JP-A 11-164912 and JP-A 2002-293996.

Of the above rare-earth catalysts, the use of a neodymium catalyst that employs a neodymium compound, which is a lanthanide series rare-earth compound, is especially recommended. In such a case, a polybutadiene rubber having a high cis-1,4 bond content and a low 1,2-vinyl bond content can be obtained at an excellent polymerization activity.

The polybutadiene has a polydispersity Mw/Mn (Mw being the weight-average molecular weight, and Mn being the number-average molecular weight) of preferably at least 2.0, more preferably at least 2.2, even more preferably at least 2.4, and most preferably at least 2.6. The upper limit is preferably 6.0 or less, more preferably 5.0 or less, and even more preferably 4.5 or less. If Mw/Mn is too low, the workability may decrease. On the other hand, if Mw/Mn is too high, the rebound may decrease.

The above polybutadiene used in the base rubber accounts for preferably not more than 80 wt %, more preferably not more than 70 wt %, even more preferably not more than 60 wt %, and most preferably not more than 57 wt %, of the overall rubber. The lower limit is preferably at least 40 wt %, more preferably at least 45 wt %, and even more preferably at least 48 wt %.

Illustrative examples of cis-1,4-polybutadiene rubbers which may be used include the high-cis products BR01, BR11, BR02, BR02L, BR02LL, BR730 and BR51, all of which are available from JSR Corporation.

In the present invention, a styrene-butadiene rubber (SBR) and/or an isoprene rubber (IR) are used together with the above polybutadiene rubber (BR). The styrene-butadiene rubber and the isoprene rubber are described below.

Isoprene rubbers (IR) which may be used include those having a cis-1,4 bond content of at least 60 wt %, preferably at least 80 wt %, and more preferably at least 90 wt %, and having a Mooney viscosity (ML₁₊₄ (100° C.)) of at least 70, preferably at least 75, and more preferably at least 80, with an upper limit of 90 or less, and preferably 85 or less. For example, the product IR2200 available from JSR Corporation may be used.

Styrene-butadiene rubbers (SBR) which may be used include solution-polymerized styrene-butadiene rubbers and emulsion-polymerized styrene-butadiene rubbers. Compared with emulsion-polymerized styrene-butadiene rubbers, solution-polymerized styrene-butadiene rubbers do not contain organic acids and low-molecular-weight components that arise from the manufacturing process, and thus have a poor processability. In addition, when a solution-polymerized butadiene-styrene rubber is used in a core-forming rubber composition, the seasonal (winter versus summer) difference in ball rebound is larger. On the other hand, when an emulsion-polymerized styrene-butadiene rubber is used in the core-forming rubber composition, compared with when a solution-polymerized styrene-butadiene rubber is used, hardening of the core due to temperature changes on account of seasonal differences can be effectively prevented. Therefore, by using these two types of styrene-butadiene rubber having different qualities in a specific ratio, it is sometimes possible to suppress a decline in resilience and a change in hardness during winter use while maintaining a good processability. By way of illustration, use may be made of the solution-polymerized products SBR-SL552, SBR-SL555 and SBR-SL563 (available from JSR Corporation) as the solution-polymerized styrene-butadiene rubber, and use can be made of the emulsion-polymerized products SBR 1500, SBR 1502, SBR 1507 and SBR 0202 (available from JSR Corporation) as the emulsion-polymerized styrene-butadiene rubber. Ordinary, commercially available, solution-polymerized styrene-butadiene rubber has a styrene bond content of from 5 wt % to 50 wt %, and emulsion-polymerized styrene-butadiene rubber has a styrene bond content of from 15 wt % to 50 wt %.

The styrene-butadiene and/or isoprene rubber account for a proportion of the overall rubber which is preferably at least 20 wt %, more preferably at least 30 wt %, even more preferably at least 40 wt %, and most preferably at least 43 wt %, but is preferably not more than 60 wt %, more preferably not more than 55 wt %, and even more preferably not more than 52 wt %.

The styrene-butadiene rubber accounts for a proportion of the overall rubber which is preferably 0 wt % or more, more preferably at least 10 wt %, and even more preferably at least 20 wt %, but is preferably not more than 60 wt %, more preferably not more than 50 wt %, even more preferably not more than 40 wt %, and most preferably not more than 35 wt %.

The isoprene rubber accounts for a proportion of the overall rubber which is preferably 0 wt % or more, more preferably at least 9 wt %, and even more preferably at least 13 wt %, but is preferably not more than 60 wt %, more preferably not more than 45 wt %, even more preferably not more than 35 wt %, and most preferably not more than 25 wt %.

In the invention, methacrylic acid is an essential ingredient which is employed as the co-crosslinking agent. Methacrylic acid is included in an amount, per 100 parts by weight of the base rubber, of preferably at least 10 parts by weight, more preferably at least 12 parts by weight, even more preferably at least 14 parts by weight, and still more preferably at least 16 parts by weight. The upper limit in the amount of methacrylic acid is preferably not more than 40 parts by weight, more preferably not more than 35 parts by weight, even more preferably not more than 30 parts by weight, and most preferably not more than 28 parts by weight. Including too much methacrylic acid may make the core too hard, giving the ball an unpleasant feel on impact. On the other hand, including too little methacrylic acid may make the core too soft, also giving the ball an unpleasant feel on impact.

It is preferable to use an organic peroxide as the crosslinking initiator in this invention. Examples of commercial products that may be advantageously used include Percumyl D, Perhexa 3M and Perhexa C40, all available from NOF Corporation. These may be used singly or as combinations of two or more thereof.

The crosslinking initiator is included in an amount, per 100 parts by weight of the base rubber, of preferably at least 0.3 part by weight, more preferably at least 0.6 part by weight, and even more preferably at least 0.9 part by weight. The upper limit in the amount of crosslinking initiator is preferably not more than 5.0 parts by weight, more preferably not more than 4.0 parts by weight, even more preferably not more than 3.0 parts by weight, and most preferably not more than 2.0 parts by weight. Including too much crosslinking initiator may make the core too hard, giving the ball an unpleasant feel on impact and also substantially lowering the durability to cracking. On the other hand, including too little crosslinking initiator may make the core too soft, giving the ball an unpleasant feel on impact and also substantially lowering productivity.

It is preferable to use zinc oxide as the metal oxide in this invention, although metal oxides other than zinc oxide may be used insofar as the objects of the invention are attainable. The metal oxide is included in an amount, per 100 parts by weight of the base rubber, of preferably at least 15 parts by weight, more preferably at least 17 parts by weight, even more preferably at least 19 parts by weight, and most preferably at least 21 parts by weight. The upper limit in the amount of metal oxide is preferably not more than 30 parts by weight, more preferably not more than 28 parts by weight, even more preferably not more than 26 parts by weight, and most preferably not more than 24 parts by weight. Including too much or too little may make it impossible to obtain a suitable weight and a suitable hardness and rebound.

In the practice of the invention, an antioxidant may also be included in the rubber composition. For example, use may be made of the commercial products Nocrac NS-6, Nocrac NS-30 and Nocrac 200, all available from Ouchi Shinko Chemical Industry Co., Ltd. These may be used singly or as combinations of two or more thereof.

The amount of antioxidant included per 100 parts by weight of the base rubber, although not subject to any particular limitation, is preferably at least 0.1 part by weight, and more preferably at least 0.2 part by weight, but is preferably not more than 1.0 part by weight, more preferably not more than 0.7 part by weight, and even more preferably not more than 0.4 part by weight. Including too much or too little antioxidant may make it impossible to achieve a suitable core hardness gradient, as a result of which a good rebound, good durability and good spin rate-lowering effect on full shots may not be achieved.

In the practice of the invention, from a resource recycling standpoint, a ground or abraded powder of s vulcanized rubber may be included in a small amount of 40 parts by weight or less per 100 parts by weight of the base rubber. In such a case, the ground or abraded powder may be compounded in an amount, per 100 parts by weight of the base rubber, which is more than 0 wt %, preferably at least 2 wt %, and most preferably at least 5 wt %, but is preferably not more than 40 wt %, more preferably not more than 35 wt %, even more preferably not more than 30 wt %, and most preferably not more than 25 wt %. The ground or abraded powder of vulcanized rubber is a vulcanizate which contains rubber and unsaturated carboxylic acid or a metal salt thereof. It is desirable for the ground or abraded powder of vulcanized rubber used to have a particle size which is preferably at least 20 μm, more preferably at least 25 μm, and most preferably at least 30 μm, but is preferably not more than 1,000 μm, more preferably not more than 900 μm, and most preferably not more than 800 μm. Adding a crushed or abraded powder of vulcanized rubber has such effects as improving the productivity of the vulcanizate and increasing the durability to cracking. However, including too much may markedly lower the workability of the rubber composition and the productivity.

The core may be produced by using a known method to vulcanize and cure the rubber composition containing the various above ingredients. For example, production may be carried out by using a mixing apparatus such as a Banbury mixer or a roll mill to mix the rubber composition, compression molding or injection molding the mixed composition in a core mold, then curing the molded body by suitable heating at a temperature sufficient for the organic peroxide and co-crosslinking agent to act, such as under conditions of between about 100° C. and about 200° C. for a period of from about 10 minutes to about 40 minutes. The core hardness profile of the invention may be achieved by a combination of the vulcanization conditions and adjustment of the rubber formulation.

The core diameter, although not subject to any particular limitation, is preferably at least 38.0 mm, more preferably at least 38.9 mm, and even more preferably at least 39.3 mm, but is preferably not more than 42.1 mm, and more preferably not more than 41.1 mm. At a core diameter outside of this range, the durability of the ball to cracking may worsen dramatically and the initial velocity of the ball may decrease.

The core has a specific gravity of preferably at least 1.05, more preferably at least 1.08, and even more preferably at least 1.1, but preferably not more than 1.2, more preferably not more than 1.15, and even more preferably not more than 1.13.

The core deflection under loading (referred to here and below as “CH”), i.e., the deflection by the core when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), is at least 2.5 mm, preferably at least 2.6 mm, and more preferably at least 2.7 mm, but is not more than 7.0 mm, preferably not more than 6.0 mm, more preferably not more than 5.5 mm, and most preferably not more than 5.2 mm. If the core deflection CH is too small, the feel of the practice golf ball on impact will be so hard as to make the ball unpleasant to use. On the other hand, if the core deflection is too large, the feel of the practice golf ball on impact will be so soft as to make the ball unpleasant to use, in addition to which the productivity may decline considerably.

The core rebound (also referred to as the core initial velocity, CV), is preferably at least 60 m/s, more preferably at least 63 m/s, even more preferably at least 66 m/s, and most preferably at least 67 m/s, but is preferably not more than 71 m/s, more preferably not more than 70.5 m/s, and even more preferably not more than 70 m/s. A core rebound outside of this range is undesirable because the distance of the ball may dramatically decline, or the ball may fly so far as to pass over the netting at a very small driving range.

In the present invention, as shown in the schematic diagram of the core in FIG. 2, letting A be the JIS-C hardness at a surface of the core, B be the JIS-C hardness at a position 2 mm inside the core surface, C be the JIS-C hardness at a position 5 mm inside the core surface, D be the JIS-C hardness at a position 10 mm inside the core surface, E be the JIS-C hardness at a position 15 mm inside the core surface, and F be the JIS-C hardness at the center of the core, it is preferable for the respective values A to F to fall within the specific ranges indicated below. By thus setting the hardness profile at the core interior within specific ranges, a comfortable feel on impact similar to that of a game ball and also a good durability to cracking can be obtained.

Letting A be the JIS-C hardness at the surface of the core, the value of A is preferably at least 60, more preferably at least 62, and even more preferably at least 63, but is preferably not more than 88, more preferably not more than 85, and even more preferably not more than 81.

Letting B be the JIS-C hardness at a position 2 mm inside the core surface, the value of B is preferably at least 54, more preferably at least 56, and even more preferably at least 58, but is preferably not more than 83, more preferably not more than 80, and even more preferably not more than 76.

Letting C be the JIS-C hardness at a position 5 mm inside the core surface, the value of C is preferably at least 56, more preferably at least 58, and even more preferably at least 60, but is preferably not more than 85, more preferably not more than 82, and even more preferably not more than 78.

Letting D be the JIS-C hardness at a position 10 mm inside the core surface, the value of D is preferably at least 54, more preferably at least 57, and even more preferably at least 60, but is preferably not more than 80, more preferably not more than 78, and even more preferably not more than 76.

Letting E be the JIS-C hardness at a position 15 mm inside the core surface, the value of E is preferably at s least 51, more preferably at least 54, and even more preferably at least 57, but is preferably not more than 75, more preferably not more than 73, and even more preferably not more than 71.

Letting F be the JIS-C hardness at the center of the core, the value of F is preferably at least 48, more preferably at least 51, and even more preferably at least 54, but is preferably not more than 72, more preferably not more than 70, and even more preferably not more than 68.

Moreover, in the above core hardness profile, it is preferable for the relative hardness conditions A>B<C≧D>E>F to be satisfied, for the value A-F to be not more than 19, for the core to be formed in such a way that A has the highest value among A to F, and for the value A-C to be in a range of from 0 to 8. If the above conditions are not satisfied, the ball may have a diminished feel on impact and a reduced durability to cracking.

The value of A-C is preferably within a range of from 0 to 8, with the lower limit being preferably more than 0, more preferably at least 1, and even more preferably at least 2, and the upper limit being preferably not more than 8, more preferably not more than 6, and even more preferably not more than 4. The value of A-F is preferably at least 2, more preferably at least 4, and even more preferably at least 6, but preferably not more than 19, more preferably not more than 18, even more preferably not more than 17, and most preferably not more than 16.

In the practice of the invention, the core may be administered surface treatment with a solution containing a haloisocyanuric acid and/or a metal salt thereof.

Prior to surface-treating the core with a solution containing a haloisocyanuric acid and/or a metal salt thereof, adhesion between the core surface and the adjoining cover material can be further enhanced by abrading the surface of the core (referred to below as “surface grinding”).

Such surface grinding removes the skin layer from the surface of the vulcanized core, and thus makes it possible to enhance the ability of the solution of haloisocyanuric acid and/or a metal salt thereof to penetrate the core surface and also to increase the surface area of contact with the adjoining cover material. Exemplary surface grinding methods include buffing, barrel grinding and centerless grinding.

The above haloisocyanuric acid and metal salt thereof is the compound shown in the following formula (I).

In the formula, X is a hydrogen atom, a halogen atom or an alkali metal atom. At least one occurrence of X is a halogen atom. Preferred halogen atoms include fluorine, chlorine and bromine, with chlorine being especially preferred. Preferred alkali metal atoms include lithium, sodium and potassium.

Illustrative examples of the haloisocyanuric acid and/or a metal salt thereof include chloroisocyanuric acid, sodium chloroisocyanurate, potassium chloroisocyanurate, dichloroisocyanuric acid, sodium dichloroisocyanurate, sodium dichloroisocyanurate dihydrate, potassium dichloroisocyanurate, trichloroisocyanuric acid, tribromoisocyanuric acid, dibromoisocyanuric acid, bromoisocyanuric acid, sodium and other salts of dibromisocyanuric acid, as well as hydrates thereof, and difluoroisocyanuric acid. Of these, chloroisocyanuric acid, sodium chloroisocyanurate, potassium chloroisocyanurate, dichloroisocyanuric acid, sodium dichloroisocyanurate, potassium dichloroisocyanurate and trichloroisocyanuric acid are preferred because they are readily hydrolyzed by water to form acid and chlorine, and thus play the role of initiating addition reactions to the double bonds in the diene rubber molecules. The use of trichloroisocyanuric acid provides an especially outstanding adhesion-improving effect.

The haloisocyanuric acid and/or a metal salt thereof is preferably dissolved in an organic solvent and used as a solution. A known organic solvent may be used for this purpose, with the use of an organic solvent which is soluble in water being especially preferred. Examples include ethyl acetate, acetone and methyl ethyl ketone. Of these, acetone is especially preferred on account of its ability to penetrate the core surface. The use of a water-soluble solvent is preferable because such solvents readily take up moisture; either the moisture which has been taken up readily undergoes a hydrolysis reaction with the haloisocyanuric acid and/or a salt thereof deposited on the core surface or, when water washing is used in a subsequent step, as the affinity to the core surface increases, a hydrolysis reaction between the water and the haloisocyanuric acid and/or a metal salt thereof more readily arises.

When dissolved in an organic solvent, the content of the haloisocyanuric acid and/or a metal salt thereof in the solution is preferably at least 0.3 wt %, more preferably at least 1 wt %, and more preferably at least 2.5 wt %. At less than 0.3 wt %, the adhesion improving effect anticipated following core surface treatment may not be obtained, possibly resulting in a poor durability to impact. The upper limit in the content may be as high as the saturated solution concentration. However, from the standpoint of cost effectiveness, when prepared as an acetone solution, for example, setting the upper limit in content to 10 wt % is preferred. The core is immersed in the solution for a length of time which is preferably at least 0.3 second, more preferably at least 3 seconds, and even more preferably at least 10 seconds, but is preferably not more than 5 minutes, more preferably not more than 1 minute, and even more preferably not more than 30 seconds. If the immersion time is too short, the desired effects of treatment may not be obtained, whereas if the immersion time is too long, a loss in ball productivity may occur.

The method of treating the core surface with a haloisocyanuric acid and/or a metal salt thereof is exemplified by methods which involve coating the surface of the core with a solution of haloisocyanuric acid and/or a metal salt thereof by brushing or spraying on the solution, and methods in which the core is immersed in a solution of the haloisocyanuric acid and/or a metal salt thereof. From the standpoint of productivity and high penetrability of the core surface by the solution, the use of an immersion method is especially preferred.

After the core has been surface treated with a solution containing haloisocyanuric acid and/or a metal salt thereof, it is preferable to wash the surface of the core with water. Water washing of the core surface may be carried out by a method such as running water, spraying, or soaking in a washing tank. However, because the aim here is not merely to wash, but also to initiate and promote the desired treatment reactions, too vigorous a washing method will not be appropriate. Therefore, preferred use may be made of washing by soaking in a washing tank. In such a case, it is desirable to place the cores to be washed from about one to five times in a washing tank that has been filled with fresh water.

Treating the core surface with a haloisocyanuric acid and/or a metal salt thereof greatly improves adhesion between the core surface and the cover. The reason for this is not well understood, but is thought to be as follows.

The haloisocyanuric acid and/or a metal salt thereof, together with the solvent, penetrates to the interior of the diene rubber making up the core and approaches the vicinity of the double bonds on the backbone. Water then enters the core surface, whereupon the haloisocyanuric acid and/or a metal salt thereof is hydrolyzed by the water, releasing the halogen. The halogen attacks the double bonds on the diene rubber backbone located nearby, as a result of which an addition reaction proceeds. In the course of this addition reaction, the liberated isocyanuric acid is added, together with the halogen, to the diene rubber backbone while retaining the cyclic structure. The added isocyanuric acid has three —NHCO— structures on the molecule.

Because —NHCO— structures are thereby conferred to the core surface that has been treated with the haloisocyanuric acid and/or a metal salt thereof, adhesion with the cover material improves further. It is most likely because of this that the durability of the golf ball to impact improves. Moreover, when a polyurethane elastomer or polyamide elastomer having the same —NHCO— structures on the polymer molecule is used as the cover material, the affinity increases even further, presumably increasing the durability to impact.

When the addition of isocyanuric acid and chlorine (as the halogen) to the surface of diene rubber has occurred, changes in the bonding states before and after addition appear in an infrared absorption spectrum as increases in the C═O bond (stretching) absorption peak at 1725 to 1705 cm⁻¹, the broad H—H bond (stretching) absorption peak at 3450 to 3300 cm⁻¹, and the C—Cl bond absorption peak at 800 to 600 cm⁻¹. Hence, by measuring the IR absorption spectrum of a surface-treated core and confirming increases in these absorption peaks, it is possible to qualitatively confirm that isocyanuric acid and chlorine addition to the diene rubber molecules at the core surface has indeed occurred.

Following surface treatment, when the material at the surface portion of the solid core is examined by differential scanning calorimetry (DSC), no exothermic or endothermic peaks are observed from room temperature to 300° C. This means that the functional groups which have been introduced maintain a stable state within this temperature range. In other words, during molding of the cover material, the functional groups which have been introduced do not undergo degradation or the like due to heat, and continue to be effective. Also, because melting in the manner of a hot melt resin does not arise, deleterious effects on durability and quality of appearance, such as resin bleed out to the parting line, do not occur. In addition, the very fact that the material in the surface portion of the solid core following the surface treatment described above is stable may be regarded as evidence that the isocyanuric acid having a melting point above 300° C. has been added with its molecular structure still intact.

Next, the material making up the cover which directly encases the core is described.

In this invention, a thermoplastic resin such as an ionomer resin or polyurethane may be used as the resin component of the cover. In particular, the use of a resin material composed primarily of polyurethane is preferred. Specifically, use may be made of a thermoplastic polyurethane elastomer or a thermoset polyurethane resin, with the use of a thermoplastic polyurethane elastomer being especially preferred.

The thermoplastic polyurethane elastomer preferably has a structure composed of soft segments formed from a polymeric polyol (polymeric glycol) and hard segments formed from a chain extender and a diisocyanate. Here, the polymeric polyol serving as a starting material may be any which has hitherto been used in the art relating to thermoplastic polyurethane materials, and is not subject to any particular limitation. Exemplary polymeric polyols include polyester polyols and polyether polyols. Polyether polyols are more preferable than polyester polyols because thermoplastic polyurethane materials having a high rebound resilience and excellent low-temperature properties can be synthesized. Illustrative examples of polyether polyols include polytetramethylene glycol and polypropylene glycol. Polytetramethylene glycol is especially preferred from the standpoint of the rebound resilience and the low-temperature properties. The polymeric polyol has an average molecular weight of preferably from 1,000 to 5,000. To synthesize a thermoplastic polyurethane material having a high rebound resilience, an average molecular weight of from 2,000 to 4,000 is especially preferred.

The chain extender employed is preferably one which has hitherto been used in the art relating to thermoplastic polyurethane materials. Illustrative examples include, but are not limited to, 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and 2,2-dimethyl-1,3-propanediol. These chain extenders have an average molecular weight of preferably from 20 to 15,000.

The diisocyanate employed is preferably one which has hitherto been used in the art relating to thermoplastic polyurethane materials. Illustrative examples include, but are not limited to, aromatic diisocyanates such as 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate is and 2,6-toluene diisocyanate, and aliphatic diisocyanates such as hexamethylene diisocyanate. Depending on the type of isocyanate, control of the crosslinking reaction during injection molding may be difficult. In this invention, the use of 4,4′-diphenylmethane diisocyanate, which is an aromatic diisocyanate, is most preferred.

A commercial product may be advantageously used as the thermoplastic polyurethane material composed of the above materials. Illustrative examples include those available under the trade names Pandex T8180, Pandex T8195, Pandex T8290, Pandex T8295 and Pandex T8260 (all available from DIC Bayer Polymer, Ltd.), and those available under the trade names Resamine 2593 and Resamine 2597 (available from Dainichiseika Color & Chemicals Mfg. Co., Ltd.).

The cover has a thickness which is preferably at least 0.3 mm, more preferably at least 0.5 mm, and even more preferably at least 0.7 mm, but is preferably not more than 2.5 mm, more preferably not more than 2.1 mm, even more preferably not more than 1.9 mm, and still more preferably not more than 1.7 mm. If the cover thickness is larger than the above range, the ball rebound will decrease, worsening the flight performance. On the other hand, if the cover thickness is smaller than the above range, the durability to cracking will decrease. In particular, when the ball is hit thin, or “topped,” the cover is likely to tear.

The cover has a specific gravity which is preferably at least 1.13, more preferably at least 1.14, and even more preferably at least 1.15, but is preferably not more than 1.30, more preferably not more than 1.20, and even more preferably not more than 1.17.

The cover material has a Shore D hardness which is preferably at least 30, more preferably at least 35, and even more preferably at least 38, but is preferably not more than 57, more preferably not more than 54, even more preferably not more than 51, and still more preferably not more than 50. If the Shore D hardness of the cover is higher than the above range, the appearance performance in long-term use (durability of markings) will decline, in addition to which the flight performance will markedly decrease. On the other hand, if the Shore D hardness of the cover is lower than the above range, the durability to cracking will greatly decrease and, particularly when the ball is topped, the cover is likely to tear. In addition, the spin rate becomes very high, shortening the distance traveled by the ball.

The practice golf ball of the invention has numerous dimples formed on the surface thereof, each dimple having a spatial volume below a flat plane circumscribed by an edge of the dimple. It is critical for the sum of the dimple spatial volumes, expressed as a percentage (VR) of the volume of a hypothetical sphere representing the ball were the ball to have no dimples on the surface thereof, to be in a range of from 0.95% to 1.7%, the lower limit being preferably 0.97%, and more preferably 0.98%, and the upper limit being preferably 1.5%, more preferably 1.3%, and most preferably 1.2%

Also, although not subject to any particular limitation, the dimples formed on the practice golf ball of the invention preferably satisfy conditions (1) and (2) below. Although it is preferable for both of the following conditions (1) and (2) to be satisfied at the same time, it is acceptable for either one of these conditions alone to be satisfied.

First, referring to FIG. 3, as condition (1), it is preferable for the dimples to have a peripheral edge provided with a roundness represented by a radius of curvature R in a range of from 0.5 mm to 2.5 mm. The lower limit of the radius of curvature R is more preferably 0.6 mm, and even more preferably 0.7 mm, and the upper limit is more preferably 1.8 mm, and even more preferably 1.5 mm.

Next, as condition (2), it is preferable for the ratio ER of a collective number of dimples RA having a radius of curvature R to diameter D ratio (R/D) of at least 20%, divided by a total number of dimples N on the surface of the ball, to be in a range of from 15% to 95%. Here, the ratio R/D is expressed as a percentage (R/D×100%), a larger value indicating a dimple in which the rounded part of the dimple accounts for a larger proportion of the dimple size and which has a smoother cross-sectional shape. The ratio ER indicates the number of such smooth dimples as a proportion of the total number of dimples; by setting ER in a range of from 15% to 95%, damage to the paint film at dimple edges can be effectively suppressed. The upper limit in the ratio R/D, although not subject to any particular limitation, is preferably not more than 60%, and more preferably not more than 40%. The lower limit in the ratio ER is more preferably 20%, and even more preferably 25%, and the upper limit is more preferably 90%, even more preferably 85%, and most preferably 70%.

Also, although not subject to any particular limitation, it is preferable for condition (3) to be satisfied. That is, as condition (3), it is preferable for the ball to have thereon a plurality of dimple types of differing diameter, and for the ratio DER of a combined number of dimples DE obtained by adding together dimples having an own diameter and an own radius of curvature larger than or equal to a radius of curvature of dimples of larger diameter than the own diameter plus dimples of a type having a largest diameter, divided by the total number of dimples N on the surface of the ball, to be at least 80%.

Generally, at a fixed dimple depth (see FIG. 3), the radius of curvature R representing the roundness provided to the peripheral edges of the dimples is smaller at smaller dimple diameters D. However, above condition (3), by such means as adjusting the depth, sets the radius of curvature R representing the roundness of the peripheral edge to be as large as possible even in dimples having a small diameter D, thus forming dimples having a smooth cross-sectional shape and, by setting the above ratio DER to at least 80%, increases the proportion of such smooth dimples, more effectively suppressing damage to the paint film. The ratio DER is more preferably at least 85%, even more preferably at least 90%, and most preferably at least 93%. The upper limit in the ratio DER is 100%.

In addition, the dimples formed on the practice golf ball of the invention, although not subject to any particular limitation, preferably satisfy conditions (4) to (6) below. Although it is preferable for all of the following conditions (4) to (6) to be satisfied at the same time, it is acceptable for any one of these conditions alone to be satisfied.

First, as condition (4), it is preferable for the number of dimple types of differing diameter D on the ball to be 3 or more, and more preferable for dimples of at least five types to be formed. In this case, the diameters D of the dimples, although not subject to any particular limitation, are preferably set in a range of from 1.5 mm to 7 mm, the lower limit being more preferably 1.8 mm and the upper limit being more preferably 6.5 mm. The depths of the dimples, although likewise not subject to any particular limitation, are preferably set in a range of from 0.05 mm to 0.35 mm, the lower limit being more preferably at least 0.1 mm, and even more preferably at least 0.13 mm, and the upper limit being more preferably not more than 0.3 mm, and even more preferably not more than 0.25 mm.

As condition (5), the total number of dimples N on the surface of the ball is preferably not more than 380, and more preferably not more than 350. The total number of dimples N is even more preferably in a range of from 220 to 340.

As condition (6), it is preferable for the dimples to be formed in such a way that the surface coverage SR of the dimples, which is the sum of individual dimple surface areas, each defined by a flat plane circumscribed by an edge of the dimple (dash-dot line in FIG. 3), expressed as a percentage of the surface area of a hypothetical sphere representing the ball were the ball to have no dimples on the surface thereof (broken line in FIG. 3), is in a range of from 60% to 74%. At a surface coverage SR greater than 74%, the intervals between neighboring dimples become too narrow, which may make it difficult to provide the dimple edges with a roundness having the radius of curvature R specified in above condition (1). On the other hand, at a surface coverage SR below 60%, the aerodynamic performance decreases, as a result of which the distance traveled by the ball may decrease. The surface coverage SR has a lower limit of more preferably 65%, and even more preferably 68%, and an upper limit of more preferably 73%.

In one-piece golf balls, because rubber has a somewhat yellow color, a white enamel paint is generally applied as a first coat, following which a clear paint is applied. In the inventive ball, in order to ensure a good appearance, it is preferable to apply a clear paint to the surface of the ball. The resulting clear coat has a thickness at dimple lands (Y) which is preferably at least 10 μm, more preferably at least 12 μm, and most preferably at least 13 μm, but is preferably not more than 30 μm, more preferably not more than 25 μm, and most preferably not more than 20 μm; and a thickness at dimple edges (Z) which is preferably at least 8 μm, more preferably at least 10 μm, and most preferably at least 11 μm, but is preferably not more than 28 μm, more preferably not more than 23 μm, and most preferably not more than 18 μm. Also, the ratio Z/Y of edge areas (Z) to land areas (Y), expressed as a percentage, is preferably at least 60%, more preferably at least 70%, and most preferably at least 80%, but is preferably not more than 100%, and more preferably not more than 95%. Outside of the above range, the durability of markings at dimple edges decreases markedly in long-term use.

The ball diameter is preferably at least 42 mm, more preferably at least 42.3 mm, and even more preferably at least 42.67 mm, but is preferably not more than 44 mm, more preferably not more than 43.8 mm, even more preferably not more than 43.5 mm, and most preferably not more than 43 mm.

The ball weight is preferably at least 44.5 g, more preferably at least 44.7 g, even more preferably at least 45.1 g, and most preferably at least 45.2 g, but is preferably not more than 47.0 g, more preferably not more than 46.5 g, and even more preferably not more than 46.0 g.

The ball has, upon initial measurement, a deflection BH1, when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), of preferably at least 2.5 mm, more preferably at least 2.6 mm, and even more preferably at least 2.65 mm, but preferably not more than 7.0 mm, more preferably not more than 6.0 mm, even more preferably not more than 5.5 mm, and most preferably not more than 5.0 mm. When the core and the ball are each compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), letting the deflections of the core and the ball be respectively CH and BH1, the ratio CH/BH1 is preferably at least 0.95, more preferably at least 0.96, and even more preferably at least 0.97, but is preferably not more than 1.1, more preferably not more than 1.09, and even more preferably not more than 1.08. If the ratio CH/BH1 is too large, the deflection of the finished ball may be too small relative to the core deflection. That is, because the cover hardness increases, the feel on impact may decrease and the quality of the appearance may decline with long-term use. On the other hand, if the ratio CH/BH1 is too small, the cover becomes very soft, which may significantly lower the durability to cracking and lead in particular to cracking of the cover when the ball is topped. In addition, the spin rate may become too high, resulting in a shorter distance of travel by the ball.

In addition, the ball has, upon initial measurement, a rebound BV1 which is preferably at least 60 m/s, more preferably at least 63 m/s, even more preferably at least 65 m/s, and most preferably at least 67 m/s, but must not be more than 70 m/s and is preferably not more than 69.8 m/s, more preferably not more than 69.6 m/s, and most preferably not more than 69.4 m/s. Outside of the foregoing range, the distance traveled by the ball may greatly decrease, or the ball may fly so far as to pass over the netting, making it unsuitable for use as a controlled-distance practice golf ball at very small driving ranges.

Moreover, in order to achieve a good durability over a long period of time, the practice golf ball of the invention, upon initial measurement, has a deflection BH1 (mm) when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) and an initial velocity BV1 (m/s) and, when measured again after being left to stand for 350 days following initial measurement, has a deflection BH2 (mm) when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) and an initial velocity BV2 (m/s), such that the difference BH2−BH1 is preferably not more than 0.2 mm, more preferably not more than 0.15 mm, and even more preferably not more than 0.1 mm, and such that the difference BV2−BV1 is preferably not more than 0.3 m/s, more preferably not more than 0.2 m/s, and even more preferably not more than 0.1 m/s.

EXAMPLES

The following Examples and Comparative Examples are provided to illustrate the invention, and are not intended to limit the scope thereof.

Examples 1 to 8, Comparative Examples 1 to 11

Rubber materials formulated as shown in Table 1 below were furnished for the fabrication of practice golf balls in the Examples and Comparative Examples. These rubber compositions were suitably mixed using a kneader or roll mill, then vulcanized under the temperature and time conditions in Table 1 to produce solid cores in the respective Examples. Ingredient amounts in the table below are shown in parts by weight.

TABLE 1 Type of Core (1) (2) (3) (4) (5) (6) (7) (8) Core BR01 55 55 55 100 60 100 95 formulation IR2200 20 15 15 5 BR730 100 BR51 40 SBR0202 25 30 30 Perhexa C-40 0.6 0.6 (40% dilution) Actual amount 0.24 0.24 of addition Percumyl D 0.9 1.1 0.9 1.2 0.8 0.6 0.6 0.8 Zinc oxide 23 23 22.5 23.5 23 9.5 9.5 23 Antioxidant 0.2 0.2 0.2 0.2 0.2 0.1 0.1 0.2 Methacrylic acid 25.5 27 17 29 18 22.5 Zinc 26 methacrylate Zinc acrylate 26 Titanium oxide 4 Vulcanization Temperature (° C.) 170 170 170 170 170 160 160 170 conditions Time (minutes) 20 20 20 20 20 13 13 30

Details on the materials used in the core formulations in the above table are provided below.

-   (1) BR01: A butadiene rubber synthesized with a nickel catalyst,     available from JSR Corporation; Mooney viscosity ML, 46 -   (2) IR2200: An isoprene rubber, available from JSR Corporation;     Mooney viscosity ML, 82 -   (3) BR730: A butadiene rubber synthesized with a neodymium catalyst,     available from JSR Corporation; Mooney viscosity ML, 55 -   (4) BR51: A butadiene rubber synthesized with a neodymium catalyst,     available from JSR Corporation; Mooney viscosity ML, 36 -   (5) SBR0202: An emulsion-polymerized styrene-butadiene rubber,     available from JSR Corporation -   (6) Perhexa C-40: An organic peroxide, available from NOF     Corporation -   (7) Percumyl D: An organic peroxide, available from NOF Corporation -   (8) Zinc oxide: Available from Sakai Chemical Co., Ltd. -   (9) Antioxidant: “Nocrac NS-6,” available from Ouchi Shinko Chemical     Industry Co., Ltd. -   (10) Methacrylic acid: Available from Kuraray Co., Ltd. -   (11) Zinc methacrylate: Available from Asada Chemical Industry Co.,     Ltd. -   (12) Zinc acrylate: Available from Nihon Jyoryu Kogyo Co., Ltd. -   (13) Titanium oxide: Available from Ishihara Sangyo Kaisha, Ltd.

In each example, after the rubber composition formulated from the ingredients shown in Table 1 was molded and vulcanized to form a core, the surface of the core was abraded to a desired diameter. Next, surface treatment of the core was carried out by immersing the core for 30 seconds in an acetone solution of trichloroisocyanuric acid (concentration, 3 wt %), then washing the surface of the core with water. The core was then set in a mold for injection molding the cover, and the cover composition shown in Table 2 below was injection molded over the solid core. Ingredient amounts in the table below are shown in parts by weight.

TABLE 2 A B C D E F G Resin Himilan 50 7331 Himilan 30 50 1557 Himilan 20 1855 Himilan 50 1601 Pandex 25 100 T8260 Pandex 100 75 25 T8195 Pandex 75 T8290 Pandex 100 T8180 Additives Mag- 1 1 nesium stearate Titanium 3.5 3.5 3.5 3.5 3.5 2.1 2.1 dioxide Poly- 1.5 1.5 1.5 1.5 1.5 ethylene wax Shore D hardness 45 50 40 58 25 50 60

Details on the materials used in the cover composition in the above table are provided below.

-   “Himilan”: Ionomer resins available under this trade name from     DuPont-Mitsui Polychemicals Co., Ltd. -   “Pandex”: Thermoplastic polyurethane elastomers available under this     trade name from Dainippon Ink & Chemicals, Inc. -   Magnesium stearate: Available from NOF Corporation -   Titanium dioxide: Available under the trade name “Tipaque R550” from     Ishihara Sangyo Kaisha, Ltd. -   Polyethylene wax: Available under the trade name “Sanwax 161P” from     Sanyo Chemical Industries, Ltd.

In order to form a predetermined dimple pattern on the surface of the cover, a plurality of protrusions corresponding to the dimple pattern were formed in the mold cavity, by means of which dimples were impressed onto the surface of the cover at the same time that the cover was injection molded. Details on the dimples are given below in Table 3. The markings shown in FIG. 5 was printed on the ball surface.

In addition, the ball was clear-coated with a paint composed of 100 parts by weight of polyester resin (acid value, 6; hydroxyl value, 168) (solids)/butyl acetate/PMA (propylene glycol monomethyl ether acetate) in a weight ratio of 70/15/15 as the base; 150 parts by weight of a non-yellowing polyisocyanate, specifically a hexamethylene diisocyanate adduct (available from Takeda Pharmaceutical Co., Ltd. as Takenate D-160N; NCO content, 8.5 wt %; solids content, 50 wt %) as the curing agent; and 150 parts by weight of butyl acetate. In Comparative Example 11, a coating of white enamel paint was applied as a base coat for clear coating.

TABLE 3 Dimple Diameter D Depth R R/D N RA ER DE DER SR VR No. Number (mm) (mm) (mm) ratio (number) (number) (%) (number) (%) (%) (%) Configuration Dimple I 1 24 4.4 0.207 0.7 16 338 102 30 330 98 72 0.99 FIG. 4 2 204 4.2 0.200 0.7 17 3 66 3.6 0.190 0.8 22 4 12 2.7 0.160 0.85 31 5 24 2.5 0.130 0.85 34 6 8 3.4 0.145 0.55 16 Dimple II 1 24 4.4 0.237 0.65 15 338 102 30 330 98 72 1.14 FIG. 4 2 204 4.2 0.230 0.65 15 3 66 3.6 0.220 0.75 21 4 12 2.7 0.190 0.8 30 5 24 2.5 0.160 0.8 32 6 8 3.4 0.145 0.45 13 Dimple III 1 24 4.4 0.216 0.5 11 338 36 11 306 91 72 0.99 FIG. 4 2 204 4.2 0.209 0.5 12 3 66 3.6 0.194 0.6 17 4 12 2.7 0.151 0.6 22 5 24 2.5 0.116 0.5 20 6 8 3.4 0.160 0.5 15

The abbreviations and symbols relating to dimples which appear in Table 3 are explained below.

-   R: Radius of curvature representing roundness provided at peripheral     edge of a dimple -   R/D ratio: Ratio of radius of curvature R to diameter D -   N: Total number of dimples on surface of ball -   RA: Collective number of dimples having an R/D ratio of at least 20% -   ER: Ratio of RA to total number of dimples N -   DE: Sum of number of dimples having an own diameter and an own     radius of curvature larger than or equal to a radius of curvature of     dimples of larger diameter than the own diameter, plus number of     dimples of a type having a largest diameter -   DER: Ratio of DE to total number of dimples N -   SR: Sum of individual dimple surface areas, each defined by a flat     plane circumscribed by an edge of the dimple, expressed as a     percentage of the surface area of a hypothetical sphere representing     the ball were the ball to have no dimples on the surface thereof -   VR: Sum of individual dimple spatial volumes, each formed below a     flat plane circumscribed by an edge of the dimple, expressed as a     percentage of the volume of a hypothetical sphere representing the     ball were the ball to have no dimples on the surface thereof.

The physical properties of the cores and covers in the respective examples of the invention and the comparative examples, and the physical properties, distance, durability and feel of the practice balls obtained in each example were measured or evaluated as described below. The results are presented in Tables 4 and 5.

Deflection of Core and Finished Ball (mm)

The deflection (mm) of the core or finished ball as the test sphere when compressed at a rate of 10 mm/min under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) was measured. The test was performed using a model 4204 test system from Instron Corporation.

Cross-Sectional Hardness of Core

The core was cut with a fine cutter and the JIS-C hardnesses at above positions B to F were measured in accordance with JIS K6301-1975 after holding the core isothermally at 23±1° C. (at two places in each of N=5 samples).

Surface Hardness of Core

JIS-C hardness measurements were carried out on the core surface in accordance with JIS K6301-1975 after holding the core isothermally at 23±1° C. (at two places in each of N=5 samples).

Rebound (Initial Velocity) of Core and Finished Ball

The initial velocity was measured using an initial velocity measuring apparatus of the same type as the USGA drum rotation-type initial velocity instrument approved by the R&A. The cores or balls used as the samples were held isothermally at a temperature of 23±1° C. for at least 3 hours, then tested in a room temperature (23±2° C.) chamber. Ten balls were each hit twice, and the time taken for the cores or balls to traverse a distance of 6.28 ft (1.91 m) was measured and used to compute the initial velocity.

Cover Material Hardness

A cover sheet was formed and, after holding the samples isothermally at 23±1° C., the Shore D hardness was measured in accordance with ASTM D-2240.

Durability to Cracking

The ball was hit thin (“topped”) five times at the same place with the leading edge of a number nine iron (X-BLADE GR, manufactured by Bridgestone Sports Co., Ltd.) at a head speed (HS) of 38 m/s, following which the ball was repeatedly struck against a wall at an incident velocity of 43 m/s and the average number of shots (N=3 balls) until cracking occurred was determined.

Abrasion Test

Ten golf balls and 3 liters of bunker sand were placed in a magnetic ball mill having an 8 liter capacity and mixing was carried out for 144 hours, following which the balls were visually examined for any loss of the markings and to assess the degree of surface scratching, the degree of loss of luster and the degree of sand adhesion. The ball appearance was rated as “good,” “fair” or “NG.”

Measurement of Coating Thickness

-   Lands (Y): The thickness of the clear coat at land areas at     intermediate positions between dimples was measured. -   Edges (Z): The thickness of the clear coat at dimple edge areas was     measured.

The above measurements were carried out at three places on each of two balls in the respective examples, and the average of these measurements was determined.

Distance

A TourStage X-Drive 701 (loft angle, 9°), manufactured by Bridgestone Sports Co., Ltd., was mounted as the driver (W#1) on a golf swing robot and struck at a head speed (HS) of 45 m/s. Both the spin rate of the ball immediately after impact and the total distance traveled by the ball were measured.

In addition, after the abrasion test described above had been carried out, the total distance of the ball was again measured.

Feel

Ten teaching professionals hit the test balls with a driver (W#1) and rated the feel of the balls on impact as good, somewhat hard (fair), or too hard (NG).

TABLE 4 Example 1 2 3 4 5 6 7 8 Core Type (1) (2) (3) (3) (2) (2) (3) (3) Diameter, mm 39.9 39.9 39.9 39.3 40.5 41.1 39.9 39.9 Specific gravity 1.118 1.118 1.118 1.118 1.118 1.118 1.118 1.118 Deflection under 10-130 kg 4 2.8 5 5 2.8 2.8 5 5 compression (CH), mm Rebound (CV), m/s 67.7 68.4 69.7 69.7 68.4 68.4 69.7 69.7 JIS-C hardness 71 79 64 64 79 79 64 64 at core surface (A) JIS-C hardness 66 74 59 59 74 74 59 59 2 mm inside core surface (B) JIS-C hardness 68 76 61 61 76 76 61 61 5 mm inside core surface (C) JIS-C hardness 66 71 61 61 71 71 61 61 10 mm inside core surface (D) JIS-C hardness 63 68 59 59 68 68 59 59 15 mm inside core surface (E) JIS-C hardness 59 64 55 55 64 64 55 55 at core center (F) JIS-C hardness difference between core 3 3 3 3 3 3 3 3 surface and 5 mm inside core (A − C) JIS-C hardness difference between core 12 15 9 9 15 15 9 9 surface and center (A − F) Cover Type A A A A A A B C Shore D hardness 45 45 45 45 45 45 50 40 Specific gravity 1.15 1.15 1.15 1.15 1.15 1.15 1.15 1.15 Thickness, mm 1.4 1.4 1.4 1.7 1.1 0.8 1.4 1.4 Finished ball Deflection under 10-130 kg 3.8 2.7 4.75 4.7 2.75 2.75 4.65 4.85 compression 30 days after production (BH1), mm Deflection under 10-130 kg 3.7 2.6 4.65 4.6 2.7 2.7 4.6 4.8 compression 350 days after measuring BH1 (BH2), mm Difference between BH1 and BH2, mm −0.1 −0.1 −0.1 −0.1 −0.05 −0.05 −0.05 −0.05 Rebound 30 days after production 67.3 67.8 69.1 68.9 68 68.2 69 69.3 (BV1), m/s Rebound 350 days after measuring 67.4 67.8 69.1 69 68 68.2 69.1 69.3 BV1 (BV2), m/s Difference between BV1 and BV2, m/s 0.1 0 0 0.1 0 0 0.1 0 Diameter, mm 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 Core initial velocity − 0.4 0.6 0.6 0.8 0.4 0.2 0.7 0.4 ball initial velocity (CV − BV1) Core deflection/ball deflection (CH/BH1) 1.05 1.04 1.05 1.06 1.02 1.02 1.08 1.03 Dimples Type I II I I II II I I Clear coating Land areas (Y), μm 15 17 15 15 17 17 15 15 thickness Edge areas (Z), μm 13 15 13 13 15 15 13 13 Coating thickness ratio 88 88 88 88 88 88 88 88 (Z/Y × 100), % Distance HS 45, driver Spin rate, rpm 3180 3360 2960 2980 3380 3480 2880 3050 (30 days after Total distance, m 194 188 192 191 189 190 192 193 production) HS 45, driver Total distance, m 191 184 189 188 185 186 187 191 (after abrasion test) Distance Total distance, m −3 −4 −3 −3 −4 −4 −5 −2 difference Durability Durability to At incident 1035 1118 930 1170 930 773 1050 788 cracking velocity of 43 m/s Abrasion test After 144 good good good good good good fair good (durability of hours of sand markings) abrasion Feel Driver good good good good good good fair good

TABLE 5 Comparative Example 1 2 3 4 5 6 Core Type (4) (5) (5) (6) (7) (7) Diameter, mm 39.9 37.3 42.3 39.9 39.9 39.9 Specific gravity 1.128 1.119 1.118 1.118 1.118 1.118 Deflection under 10-130 kg 1.8 3.8 3.8 3.8 3.8 3.8 compression (CH), mm Rebound (CV), m/s 73.7 74.7 74.7 77 77.4 77.4 JIS-C hardness 90 59 69 70 70 70 at core surface (A) JIS-C hardness 88 63 63 65 65 65 2 mm inside core surface (B) JIS-C hardness 87 67 67 68 68 68 5 mm inside core surface (C) JIS-C hardness 81 66 66 66 66 66 10 mm inside core surface (D) JIS-C hardness 74 64 64 62 62 62 15 mm inside core surface (E) JIS-C hardness 70 61 61 58 58 58 at core center (F) JIS-C hardness difference between core 3 2 2 2 2 2 surface and 5 mm inside core (A − C) JIS-C hardness difference between core 20 8 8 12 12 12 surface and center (A − F) Cover Type A A A A A G Shore D hardness 45 45 45 45 45 60 Specific gravity 1.15 1.15 1.15 1.15 1.15 0.99 Thickness, mm 1.4 2.7 0.2 1.4 1.4 1.4 Finished ball Deflection under 10-130 kg 1.9 3.4 3.9 3.8 3.8 3.3 compression 30 days after production (BH1), mm Deflection under 10-130 kg 1.9 3.4 3.9 3.5 3.5 3 compression 350 days after measuring BH1 (BH2), mm Difference between BH1 and BH2, mm 0 0 0 −0.3 −0.3 −0.3 Rebound 30 days after production 74.4 73.4 74.7 76 76.4 77.0 (BV1), m/s Rebound 350 days after measuring 74.4 73.4 74.8 75.1 75.5 76.6 BV1 (BV2), m/s Difference between BV1 and BV2, m/s 0 0 0.1 −0.9 −0.9 −0.4 Diameter, mm 42.7 42.7 42.7 42.7 42.7 42.7 Core initial velocity − −0.7 1.3 0.0 1.0 1.0 0.4 ball initial velocity (CV − BV1) Core deflection/ball deflection (CH/BH1) 0.95 1.12 0.97 1.00 1.00 1.15 Dimples Type I III I I I I Clear coating Land areas (Y), μm 15 17 15 15 15 15 thickness Edge areas (Z), μm 13 8 13 13 13 13 Coating thickness ratio 88 47 88 88 88 88 (Z/Y × 100), % Distance HS 45, driver Spin rate, rpm 3480 2830 3400 3100 3070 3040 (30 days after Total distance, m 223 210 217 224 226 228 production) HS 45, driver Total distance, m 219 203 214 221 223 213 (after abrasion test) Distance Total distance, m −4 −7 −3 −3 −3 −15 difference Durability Durability to At incident 1425 1680 585 555 518 300 cracking velocity of 43 m/s Abrasion test After 144 good NG good good good NG (durability of markings) hours of sand abrasion Feel Driver NG NG good good good good Comparative Example 7 8 9 10 11 Core Type (7) (5) (5) (5) (8) Diameter, mm 39.9 39.9 39.9 39.9 42.7 Specific gravity 1.118 1.118 1.118 1.118 1.121 Deflection under 10-130 kg 3.8 3.8 3.8 3.8 compression (CH), mm Rebound (CV), m/s 77.4 74.7 74.7 74.7 JIS-C hardness 70 69 69 69 80 at core surface (A) JIS-C hardness 65 63 63 63 75 2 mm inside core surface (B) JIS-C hardness 68 67 67 67 77 5 mm inside core surface (C) JIS-C hardness 66 66 66 66 71 10 mm inside core surface (D) JIS-C hardness 62 64 64 64 67 15 mm inside core surface (E) JIS-C hardness 58 61 61 61 63 at core center (F) JIS-C hardness difference between core 2 2 2 2 3 surface and 5 mm inside core (A − C) JIS-C hardness difference between core 12 8 8 8 17 surface and center (A − F) Cover Type A D E F Shore D hardness 45 58 25 50 Specific gravity 1.15 1.15 1.15 0.99 Thickness, mm 1.4 1.4 1.4 1.4 Finished ball Deflection under 10-130 kg 3.8 3.40 3.9 3.6 3.1 compression 30 days after production (BH1), mm Deflection under 10-130 kg 3.5 3.4 3.9 3.5 3.1 compression 350 days after measuring BH1 (BH2), mm Difference between BH1 and BH2, mm −0.3 0.0 0 −0.1 0 Rebound 30 days after production 76.4 73.9 74.2 73.5 74.6 (BV1), m/s Rebound 350 days after measuring 75.5 74.0 74.2 73.6 74.7 BV1 (BV2), m/s Difference between BV1 and BV2, m/s −0.9 0.1 0 0.1 0.1 Diameter, mm 42.7 42.7 42.7 42.7 42.7 Core initial velocity − 1.0 0.8 0.5 1.2 ball initial velocity (CV − BV1) Core deflection/ball deflection (CH/BH1) 1.00 1.12 0.97 1.06 Dimples Type III I I I I Clear coating Land areas (Y), μm 17 15 15 15 15 thickness Edge areas (Z), μm 8 13 13 13 13 Coating thickness ratio 47 88 88 88 88 (Z/Y × 100), % Distance HS 45, driver Spin rate, rpm 3070 2900 3520 3110 3650 (30 days after Total distance, m 226 214 209 211 221 production) HS 45, driver Total distance, m 219 206 207 199 215 (after abrasion test) Distance Total distance, m −7 −8 −2 −12 −6 difference Durability Durability to At incident 518 1305 612 960 621 cracking velocity of 43 m/s Abrasion test After 144 NG NG good NG NG (durability of markings) hours of sand abrasion Feel Driver good NG good good good

In the practice golf ball of Comparative Example 1, the deflection of the finished ball was too small. As a result, the ball had too hard a feel on impact, making it uncomfortable to use as a practice golf ball.

In the practice golf ball of Comparative Example 2, the cover was too thick, as a result of which the ball had a dull feel on impact. In addition, the radius of curvature at the dimples edges was small and the edge-to-land coating thickness ratio was small, as a result of which the durability of markings was very poor. Moreover, the flight performance following the abrasion test (following evaluation of the durability of markings) decreased.

In the practice golf ball of Comparative Example 3, the cover was too thin. As a result, the durability to cracking was poor, with the durability to topping being particularly poor.

In the practice golf ball of Comparative Example 4, zinc methacrylate was included as the co-crosslinking agent in the core-forming rubber formulation. As a result, the durability to cracking was poor.

In the practice golf ball of Comparative Example 5, zinc acrylate was included as the co-crosslinking agent in the core-forming rubber formulation. As a result, the durability to cracking was very poor.

In the practice golf ball of Comparative Example 6, zinc acrylate was included in the rubber composition and the cover was composed primarily of an ionomer. As a result, the durability to cracking was very poor. In addition, because the cover was composed primarily of an ionomer, it was too hard, as a result of which the durability of markings was very poor and the flight performance following the abrasion test (following evaluation of the durability of markings) greatly decreased.

In the practice golf ball of Comparative Example 7, zinc acrylate was included, as a result of which the durability to cracking was very poor. In addition, the radius of curvature at the dimple edges was small and the edge-to-land coating thickness ratio was small, as a result of which the durability of markings was very poor. Moreover, the flight performance following the abrasion test (following evaluation of the durability of abrasions) decreased.

In the practice golf ball of Comparative Example 8, the cover was too hard, as a result of which the durability of markings was very poor. In addition, the flight performance following the abrasion test (following evaluation of the durability of markings) greatly decreased.

In the practice golf ball of Comparative Example 9, the cover was too soft, as a result of which the durability to cracking was poor. When the ball was topped in particular, the cover ended up cracking. In addition, the spin rate was too high, reducing the distance traveled by the ball.

In the practice golf ball of Comparative Example 10, the cover was formed by using an ionomer as the resin component. As a result, the durability of markings was very poor. In addition, the flight performance following the abrasion test (following evaluation of the durability of markings) greatly decreased.

The practice golf ball of Comparative Example 11 was a one-piece golf ball composed of a single layer. As a result, the durability to cracking was poor and the spin rate was high. In addition, the decrease in the flight performance following the abrasion test (following evaluation of the durability of markings) was somewhat large.

Japanese Patent Application No. 2011-099969 is incorporated herein by reference.

Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims. 

1. A practice golf ball comprising: a core made of a rubber composition comprising a base rubber and, as compounding ingredients: a co-crosslinking agent, a crosslinking initiator and a metal oxide; and a cover which encases the core and comprises a resin component, wherein the base rubber is a mixture of polybutadiene with styrene-butadiene rubber or isoprene rubber or both; the co-crosslinking agent is methacrylic acid; the core, when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), has a deflection CH of from 2.5 mm to 7.0 mm; the ball has an initial velocity of not more than 70 m/s; and the ball has formed on a surface thereof a plurality of dimples, each dimple having a spatial volume below a flat plane circumscribed by an edge of the dimple, and the sum of the dimple spatial volumes, expressed as a percentage (VR) of the volume of a hypothetical sphere representing the ball were the ball to have no dimples on the surface thereof, being from 0.95% to 1.7%.
 2. The practice golf ball of claim 1, wherein the core has a hardness profile in which, letting A be the JIS-C hardness at a surface of the core, B be the JIS-C hardness at a position 2 mm inside the core surface, C be the JIS-C hardness at a position 5 mm inside the core surface, D be the JIS-C hardness at a position 10 mm inside the core surface, E be the JIS-C hardness at a position 15 mm inside the core surface, and F be the JIS-C hardness at a center of the core: A is from 60 to 88, B is from 54 to 83, C is from 56 to 85, D is from 54 to 80, E is from 51 to 75, and F is from 48 to 72; the relative hardness conditions A>B<C≧D>E>F are satisfied; the value A-F is not more than 19; the core is formed in such a way that A has the highest value among A to F; the value A-C is from 0 to 8; the core has a specific gravity of from 1.05 to 1.2; the resin component of the cover is composed primarily of polyurethane; the cover has a material hardness, expressed as the Shore D hardness, of from 30 to 57; the cover has a thickness of from 0.3 mm to 2.5 mm; and when the core and the ball are each compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), letting deflection by the core be CH and deflection by the ball be BH1, the ratio CH/BH1 is from 0.95 to 1.1.
 3. The practice golf ball of claim 1, wherein the ball has, upon initial measurement, a deflection BH1 (mm) when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) and an initial velocity BV1 (m/s), and also has, when measured again after being left to stand for 350 days following initial measurement, a deflection BH2 (mm) when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) and an initial velocity BV2 (m/s), such that the difference BH2−BH1 is not more than 0.2 mm and the difference BV2−BV1 is not more than 0.3 m/s.
 4. The practice golf ball of claim 1, wherein the ball has formed on a surface thereof a plurality of dimples which satisfy conditions (1) and (2) below: (1) the dimples have a peripheral edge provided with a roundness represented by a radius of curvature R of from 0.5 mm to 2.5 mm; and (2) the ratio ER of a collective number of dimples RA having a radius of curvature R to diameter D ratio (R/D) of at least 20%, divided by a total number of dimples N on the surface of the ball, is from 15% to 95%.
 5. The practice golf ball of claim 4 which further satisfies condition (3) below: (3) the ball has thereon a plurality of dimple types of differing diameter, and the ratio DER of a combined number of dimples DE obtained by adding together dimples having an own diameter and an own radius of curvature larger than or equal to a radius of curvature of dimples of larger diameter than said own diameter plus dimples of a type having a largest diameter, divided by the total number of dimples N on the surface of the ball, is at least 80%.
 6. The practice golf ball of claim 5 which further satisfies conditions (4) to (6) below: (4) the number of dimple types of differing diameter is 3 or more; (5) the total number of dimples N is not more than 380; and (6) the surface coverage SR of the dimples, which is the sum of individual dimple surface areas, each defined by a flat plane circumscribed by an edge of the dimple, expressed as a percentage of the surface area of a hypothetical sphere representing the ball were the ball to have no dimples on the surface thereof, is from 60% to 74%.
 7. The practice golf ball of claim 1, wherein the polyurethane in the resin component of the cover is a thermoplastic polyurethane elastomer.
 8. The practice golf ball of claim 7, wherein the thermoplastic polyurethane elastomer comprises soft segments formed from a polymeric polyether polyol and hard segments formed from an aromatic diisocyanate.
 9. The practice golf ball of claim 1, wherein the ball has, upon initial measurement, a deflection BH1 when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) of from 2.5 mm to 7.0 mm.
 10. The practice golf ball of claim 1 wherein, in the rubber composition making up the core, polybutadiene accounts for not more than 80 wt % of the base rubber, styrene-butadiene rubber or isoprene rubber or both account for from 20 wt % to 60 wt % of the base rubber, and the compounding ingredients are included in respective amounts of from 10 to 40 parts by weight of methacrylic acid, from 15 to 30 parts by weight of metal oxide, from 0.3 to 5.0 parts by weight of crosslinking initiator, and from 0.1 to 1.0 part by weight of an antioxidant, per 100 parts by weight of the base rubber. 